Historically, since the development of jet engines immediately following World War II, the operating temperature of the engines has been increased by different technical developments by about 15.degree. F. per year. The present invention relates to an improvement in the temperature capabilities of materials used in the jet engines of about 450.degree. F. This represents about a 30-year improvement in the operating temperature of jet engines. The gains these materials offer are about equivalent to the gains achieved from 1959 to 1989.
This scale of improvement in jet engine performance makes the subject invention, and the applications cross-referenced above, candidate inventions for use in the U.S. Air Force, Integrated High Performance Turbine Engine (IHPTE), propulsion initiative under which the Air Force seeks to double the thrust to weight ratio of a newly designed generation of jet engines over the most advanced production engines which are available today.
The most efficient production engine flying today has a thrust to weight ratio of about 10 to 1. The propulsion initiative of the Air Force is a proposed objective to bring that ratio to about 20 to 1.
The maximum metal temperatures of presently employed jet engines are about 2200.degree. F. Materials which are presently used in jet engines which operate in the neighborhood of 2200.degree. F. are molten at the temperatures above .about.2450.degree. F. The maximum metal temperature of jet engines employing the materials of the present invention can approach 2700.degree..
Another factor which suggests the utility of the materials system of the present invention in the Air Force propulsion initiative is that some of the materials have a density reduction of more than 20% when compared to the materials used in production engines today. Presently used nickel base superalloys have densities ranging from about 0.295 to 0.320 pounds per cubic inch and average over 0.300. Several materials of the materials systems of the present invention have densities less than 0.235.
In other words, there are materials in the materials systems of the present invention which have lower density than the nickel-base superalloy material presently employed in the formation of jet engines and there are materials of these systems which may operate at significantly higher temperature and, in fact, well above the temperature at which nickel base alloys are molten.
The lower density of the materials of fabrication of the engine is a very desirable property of these materials inasmuch as the use of lower density materials results in the engine operating with the desirable higher thrust to weight ratio as compared to the present generation of engines. The use of lower density materials is particularly important in the rotating parts of engines. Such rotating parts rotate at about 12,000 revolutions per minute and, accordingly, very high centrifugal forces are generated in the rotating parts. By reducing the density of the materials in the rotating parts of the engine without reducing their strength and toughness, the actual mass of material which must be built into such a rotating part can be greatly reduced. The increase in the thrust to weight ratio can accordingly be larger than a simple proportional reduction in density where the density of the material of which the engine is formed is decreased from the density of presently employed materials.
In present production engines rotating parts normally do not operate at temperatures above about 1900.degree. F. An object of the present invention is to provide materials which can be incorporated into rotating parts for operation at significantly higher temperatures of the order of 2300.degree. F. and/or which can operate at lower densities and provide a significant mass savings.
The higher operating temperature of a jet engine employing the materials system of the present invention has a number of advantages and benefits.
One advantage is that the burning of fuel in the engine is more complete and, therefore, more efficient. The burning is more complete because it is at a higher temperature. A complete or stoichiometric burning of fuel can produce a flame temperature of over 4000.degree. F. Stoichiometric burning is avoided because such a temperature produced in an engine would require too much cooling to avoid material temperatures so great that essentially all materials in present production engines would melt. While the potential flame temperature of a future jet engine may reach 4000.degree. F., the actual operating temperature of the hotter metal parts of a present production jet engine is about 2000.degree. F. to 2200.degree. F. This lower metal temperature is maintained, although the actual flame temperature is higher, through a complex set of cooling air flow schemes within the engine to protect the metal parts from the higher temperatures. This air flow for cooling engine parts requires a lot of special plumbing and reduces engine efficiency both because of the lower operating temperatures and because of the need for extensive cooling air flow and its attendant weighty plumbing. An engine which can operate with hotter metal parts gains in efficiency both from increased operating temperature and from reductions in cooling air flow and associated plumbing.
The materials employed in the present invention are components of a materials system. The projected gain in operating temperature of a jet engine according to the present invention is the result of use of a different material system than has been used heretofore. The system involves two or more distinct elements, each of which has a different composition and each of which performs a different function in the operation of the system within the materials structures of the jet engine. The two basic elements of the system are, first, a lighter weight metallic substrate which provides structural capability for the system, and, second, a metallic coating material which protects the substrate from environmental attack. Other elements may be included. One such element of the system may be an overcoating of a non-metallic character.
Different parts of an engine operate at different temperatures. The combination of substrate and coating are selected pursuant to the present invention to suit the material needs of specific engine parts.
Not all materials of an engine need be at the highest temperature. The coating materials of copending application Ser. No. 214,078, filed July 7, 1988, and particularly the RuFeCrAlY material can operate in air without substantial loss of coating material for extended periods at temperatures of 2750.degree. F. and higher. In other words, the metallic coating material of copending application Ser. No. 214,078 is capable of protecting metallic substrates from oxidative attack to temperatures of 2750.degree. F. and higher. This is a reference temperature for use of the materials of the materials systems of the present invention.
A wide variety of substrate materials are disclosed in the copending applications cross-referenced above and these substrate materials may be coated with the coating materials of Ser. No. 214,078.
Individual substrate materials and their individual distinctive properties are disclosed in the copending applications referenced above and they can be used in conjunction with protective coating materials such as the RuCrAlY and RuFeCrAlY materials of copending application Ser. No. 214,078 referred to above. These substrate materials are all ductile alloy materials and are not brittle intermetallic compounds as many high temperature materials are.
There are several substrate materials of different compositions which are suitable for use with coatings such as the RuCrAlY and RuFeCrAlY coatings of Ser. No. 214,078. Not all of the substrates have the same compositions or the same density or the same melting point or the same tensile properties at high use temperatures. In treating these combinations of materials as a system, advantage can be taken of the variety of high temperature alloy materials disclosed in the referenced copending applications to provide a combination of substrate and coating materials which best suit a specific use application as for example a specific part such as a vane, within a jet engine. For example, the variety of materials disclosed in the several copending applications referenced above makes it possible to provide materials and material combinations to satisfy a number of different temperature, density, strength and related materials criteria. The materials criteria of materials for use in rotating parts is not the same as the criteria for use in stationary parts. Similarly, the temperature at which a material is to be used is influential both in selection of the substrate as well as in selection of a coating for the substrate if one is to be used. Moreover, it is realized that the materials of the systems of the present invention are particularly suitable for the higher temperature portions of a jet engine. Other materials such as titanium base alloys will be preferred for use in lower temperature portions of the engine such as in the early stage compressor blades of the engine.